In the last 20 years, the Hubble Space Telescope has
revolutionized the way humanity views the universe. In many ways, it may have
been the most influential telescope since Galileo peered at the night sky with
one four centuries ago.

The greatest insights often make the world seem like a larger
place than it was before. In Hubble's case, the most important and perhaps most
confounding discovery it helped find accomplished just that, by revealing the
universe was expanding faster than anyone had known. [New
Hubble photos.]

NASA launched the Hubble Space Telescope, a joint effort by NASA and the
European Space Agency, on April 24, 1990 aboard the space shuttle Discovery to
much fanfare that soon fell flat. A flaw in the telescope's optics gave it blurry
vision and turned the iconic space telescope into a potential boondoggle in
orbit.

But Hubble was built to be upgraded by astronauts riding NASA
shuttles. In 1993, the first crew of space mechanics fixed the Hubble telescope's
vision flaw, with four more maintenance and repair missions to follow.

NASA's last trip to Hubble
was in May 2009, when the crew of shuttle Atlantis paid one final service call
to the orbital observatory. They replaced Hubble's old batteries and worn out parts,
revived broken cameras never designed to be fixed in space and added two new
instruments. The result: A Hubble Space Telescope more powerful than ever.

Here's a look at some of Hubble's greatest astronomical
achievements:

Hubble's greatest discovery

Scientists have dubbed the suspected culprit behind this
accelerated expansion "dark energy," and it is now thought to make up
74 percent of the combined mass-energy in the entire universe. In comparison,
ordinary matter accounts for only 4.6 percent.

"The discovery of dark energy was extremely surprising, and
is I think the greatest discovery it helped make," said astrophysicist
Mario Livio at the Space Telescope Science Institute, the science
operations center for the Hubble
Space Telescope. "And we still don't have any idea really what it is.
The nature of this dark energy is at some level the biggest problem
that physics is facing today."

And Hubble did not only make the universe seem a larger place by
showing us it was growing ? the orbiting telescope also hinting there was a lot
more for us to learn.

"It's
generated a tremendous sense of humility because we've discovered how we
understand so little about the universe, from dark energy to dark matter to how
galaxies can change across 13 billion years of cosmic history," said Space
Telescope Science Institute director Matt Mountain. "It's completely changed
our perspective on the universe."

A surprising find

When Hubble was launched, one of its main missions was discovering
when the universe was born. Before the orbiting telescope was deployed, it was
highly uncertain as to how old the universe was, which could lead to laughable
possibilities, such as stars older than the universe.

By
measuring where distant galaxies are more accurately than ever before and how
fast they are moving, Hubble greatly narrowed down the rate at which the
universe is expanding, helping refine estimates of the universe's age down to
roughly 13.75 billion years. However, by solving the mystery of the universe's
age, it unexpectedly turned up an even more profound enigma ? the universe's
expansion is inexplicably accelerating, instead of slowing down as one might expect
due to the pull of gravity from galaxies.

There were earlier suggestions that a "cosmological
constant" might exist with the effect of a repulsive force that acted
against matter's gravitational attraction, with the most notable proposal
coming from Einstein. Before Hubble, however, "without observations, no
one took those speculations particularly seriously," Livio said.

The promise of dark energy

Solving the mystery of dark
energy could revolutionize physics. It has prompted new theories regarding the
origin of the universe, such as one where clashing membranes of reality trigger
endless cycles of cosmic death and rebirth. It has also prompted speculation
regarding the universe's fate, raising the possibility that dark energy ends
the universe in a Big Rip.

Still, much remains unknown about dark energy. One idea is that it
literally comes from empty space ? from energy that quantum mechanics theorizes
should exist in vacuum.

The problem is that preliminary calculations as to how strong dark
energy might be if it was a consequence of vacuum energy were an astounding 120
orders of magnitude greater than we actually see with dark energy. That is a 1
with 120 zeroes behind it.

"Even if you refine estimates further, you still miss the
mark by more than 50 orders of magnitude, which is again ridiculous,"
Livio said. "Another possibility is that it's some sort of field, but we
don't understand why that field should be there, and whether it might be
related in some fashion to what caused inflation of the universe at its very
beginning. A third possibility is that there's really no dark energy at all,
but that we have to change our theory of gravity, that Einstein's theory of
general relativity is not correct when we get to larger scales of the
universe."

In each of those cases, "we're talking about a fundamental
change in our understanding of physics, the very basic physical theory that
governs the universe," Livio noted.

Not since Galileo

"It's
fair to say that when you look back at history, Hubble will have had as much
impact as Galileo's
telescope," Mountain said. "No telescope has had the kind of
public draw that Hubble's had. It's the way the public gets to participate.
Everyone gets to see its pictures ? you don't have to know how to read Latin to
read Galileo's 'Starry Messenger' to find out what he was up to."

A
great deal of Hubble's revolutionary impact comes from its staying power.
"It's been serviced five times, and each such mission has allowed Hubble
to renew itself with new instruments that make it almost a new telescope each
time, so that it can keep making new discoveries," Livio said.

After
the last mission that serviced Hubble, "we expect it to last at least
another five years," Mountain said. "In principle, if we're lucky and
smart, we may be able to celebrate Hubble's 30th birthday."